The drying of large quantities of particulate or powdery material to a desired moisture content is difficult especially for heat sensitive materials such as grain. Without uniform drying, degradation of heat sensitive materials may occur in the hot areas of the dryer, while other areas of the dryer may not sufficiently dry the particulate material without prolonged drying times. There is a balance therefore between the drying time and the drying temperature that is used to dry the particulate material that must be met, particularly if the particulate material is prone to degradation from overheating. For example, if grain is heated too quickly and then quickly cooled during the drying operation, the sudden temperature changes may tend to cause stress cracking and shattering of the grain. Such cracking or shattering of the grain greatly lowers the value of the grain such that it may not be acceptable to many grain elevators and processors.
In some instances, uniformity of the moisture content of the dried particulate material is also important. Excessive moisture in a portion of the particulate material after drying may present problems with handling, particularly if the material is prone to agglomeration in the presence of moisture. Furthermore, the presence of undesirable moisture may increase the corrosion rate of storage vessels containing particulate materials such as halogenated catalysts and the like. In all of the above examples, uniform drying of particulate material to a desired moisture level is an important consideration.
Many systems have been developed over the years which are intended to heat and uniformly dry particulate material such as grain while at the same time avoiding problems associated with drying heat sensitive materials. One such system is the cross-flow column type particulate dryer in which a heated gas is forced mainly in a transverse direction through a downward moving bed of particulate material to dry the material. Typically the hot gas enters the dryer from an inlet side and travels from the inlet gas side to an opposing side of the dryer while at the same time passing through a foraminous wall into the particulate material to be dried. One means for distributing the gas from the inlet side to the opposing side of the dryer is generally disclosed in U.S. Pat. Nos. 4,398,356; 4,423,557; and 4,424,634 to Westelaken.
Counter flow drying systems have also been used to dry particulate materials. In this case, heated gas is forced through the particulate material in a direction opposite to the direction of flow of the material through the dryer. This method of drying, however, may result in overdrying the material near the hot gas inlet thus leading to product degradation. While counter flow dryers may be more efficient overall, they are not well suited to the drying of heat sensitive materials.
Another type of dryer that may be used for heat sensitive materials is the concurrent-countercurrent flow dryer in which heated gas travels through the particulate material in the same direction as the moving bed of particulate material and a countercurrent flow of cooling gas travels in a direction opposite to the direction of flow of the particulate material. In this system, gas exhaust means may be provided intermediate the hot gas inlet and cooling gas inlet of the dryer. With such a system, the hottest gas is available at a point in the dryer wherein the particulate material is the wettest and coolest. As the particulate material and hot gas travel together, the gas heats and dries the particulate material while the particulate material gradually cools the gas. The counter flow of cooling gas serves to further cool and temper the particulate material before it reaches the outlet of the dryer.
A particular useful concurrent-countercurrent flow dryer for particulate material is disclosed in U.S. Pat. No. 4,086,708 to Westelaken. In the Westelaken system, the efficiency of the dryer is based, at least in part, on the use of drying temperatures in excess of 500.degree. F. (260.degree. C.) near the hot gas inlet. Accordingly, careful control over the flow of particulate material and hot gas are essential to obtain a uniform rate of particulate drying across the drying chamber. Any blockages in the flow of the particulate material or channeling of the gas in the bed of particulate material must be avoided, particularly for heat sensitive materials.
With all of the above, distribution of a drying gas such as air, is a particular problem for large scale particulate dryers. In a large scale particulate dryer, the drying gas may have to travel an extended distance from the inlet gas plenum to the opposing side of the dryer. Distribution of the drying gas evenly throughout the particulate material becomes increasingly more difficult the greater the distance the gas must travel prior to contacting the particulate material to be dried. Smaller dryers which may have the ability to more readily distribute the drying gas, do not provide the drying capacity and thus economies of scale that larger dryers provide.
It is therefore one object of the invention to provide an improved inlet gas distributor for a particulate dryer.
It is another object of the invention to provide an inlet gas distributor for evenly distributing gas diagonally across a path of gas flow so that the gas will more efficiently contact particulate material to be dried in a particulate material dryer.
Still another object of the invention is to provide a plurality of gas deflectors arranged diagonally across a path of longitudinal gas flow for evenly distributing a gas along the longitudinal path of gas flow so that the gas exiting the distributor is directed in an essentially perpendicular direction to the direction of gas flow entering the distributor.
These and other object of the invention will be evident from the ensuing description and appended claims.